Spectra of Extra-Solar Planet Puzzles Astronomers

California Institute of Technology News Release

2007 February 21

PASADENA, Calif.--A team of astronomers led by Carl Grillmair of the California
Institute of Technology has discovered some puzzling things about a
Jupiter-sized planet that passes in front of a nearby star in the constellation
Vulpecula.

Both the Grillmair team and groups from NASA's Goddard Space Flight Center and
NASA's Jet Propulsion Laboratory are reporting today on their independent
findings about two transiting exoplanets. These are the first spectra from
planets outside our own solar system, and have been made possible by the NASA
Spitzer Space Telescope's unexpectedly keen ability to study nearby stars.

According to Grillmair, an astronomer at Caltech's Spitzer Science Center, the
planet studied by his group is named HD 189733b. The planet is about 62
light-years, or 360 trillion miles away from Earth, is about 10 percent larger
than Jupiter, and has a "year" that lasts only two days. It orbits the star HD
189733, which is somewhat smaller and slightly redder than our own sun. And
unexpectedly, the data doesn't show the presence of water.

"It's surprising," says Grillmair. "According to what the theoreticians tell us,
we had expected to see a very structured spectrum that would have a particular
shape because of the presence of water in the planet's atmosphere. But what we
actually see is a very flat spectrum."

Spectral data is good for determining what's in a star--or planet, for that
matter--because different substances can look very different when the light from
them is split into separate colors by a prism. Scientists in the 19th century
discovered that burning a substance and then looking at its light through a
prism was an excellent way of figuring out what was being burned, and roughly
the same procedure has been used ever since for finding out about the
light-emitting things in the universe.

The problem with exoplanets, however, has been that the light of the star can be
billions of times brighter than the planet itself. As a result, astronomers have
previously been unable to study the spectra of planets outside our solar system
due to the sheer distance and their inability to distinguish planet light from
starlight.

"Normally, trying to see a planet next to a star is like trying to see a firefly
next to an airport searchlight several miles away," Grillmair explains. "But in
the case of our planet and the one being reported by the other teams, you can
take the combined spectrum of the star and planet, and then when the planet
passes behind the star, take another spectrum. By subtracting the second
spectrum of just the star from the first, you can divine the spectrum of the
planet itself."

Another key element to this discovery is that the observations are done in the
infrared. The contrast between the star and the planet isn't as large in the
infrared, so researchers can tease out the infrared spectrum of the planet. It
remains impossible, with current technology, to do this in the visible light,
even for transiting planets.

As for the apparent lack of water, Grillmair says there are at least four
possibilities. First of all, there could really be no water, which he feels is
not very likely. Second, there could be some other chemicals in the planet's
atmosphere that emit radiation just where water absorbs it, thereby effectively
camouflaging the signature of the water. This too seems unlikely. Third, the
water could be hidden underneath an opaque cloud layer the Spitzer telescope
can't see through. Fourth, a theoretical model suggests that, if the planet is
in tidal lock (in other words, is so close to its sun that the same side always
faces the same way), the atmospheric temperature profile on the day-side of the
planet could be such that spectral features are suppressed.

But whatever the case, Grillmair thinks that a healthy collection of additional
data during the Spitzer's final year or two of life could settle the matter--and
teach us much about the worlds beyond our solar system.

"We really need more data to hammer this thing and knock down the noise," he
says. "There will be 17 eclipses during the next year that will be visible to
Spitzer, and I'd really like to look at every one of them."

So far, Grillmair and his team have been able to observe the planet for a total
of 12 hours during two eclipses. A nearly tenfold increase in data would allow
positive identifications of individual chemical elements, which has not been
possible with the data returned so far.

"This type of data will undoubtedly be one of Spitzer's greatest legacies,"
Grillmair says. "Transiting extrasolar planets hadn't even been discovered when
the Spitzer Space Telescope was designed, so this was all unanticipated."

NASA's Jet Propulsion Laboratory, located in Pasadena, California, manages the
Spitzer Space Telescope mission for NASA's Science Mission Directorate,
Washington. Science operations are conducted at the Spitzer Science Center at
the California Institute of Technology, also in Pasadena. Caltech manages JPL
for NASA.

The other members of Grillmair's team are David Charbonneau of the
Harvard-Smithsonian Center for Astrophysics; Adam Burrows of the Steward
Observatory; Lee Armus, John Stauffer, Victoria Meadows, and Deborah Levine, all
of the Spitzer Science Center; and Jeffrey Van Cleve of Ball Aerospace and
Technologies Corp.

The Grillmair team's results will be published in an upcoming issue of
Astrophysical Journal Letters. A report on the Goddard Space Flight Center
team's study of the transiting exoplanet HD 209458b is being published this week
in the journal Nature.

A separate paper by the JPL-led team on HD 209458b has been submitted to the
Astrophysical Journal Letters. The JPL team, led by Mark Swain, also includes
Caltech's Rachel Akeson and Chas Beichman

This story was originally titled "Astronomers Puzzled by Spectra of Transiting Planet Orbiting Nearby Star".